Pocketed coil springs, i.e. coil springs enclosed within pockets of fabric which are joined at their side seams, are widely used in the manufacture of mattresses, cushions and the like. Apparatus for the production of pocketed coil springs generally comprises a coiling assembly in which a coil spring is formed from wire, and an encapsulation assembly in which the coil spring is encapsulated within a pocket. In particular, the coil spring is generally formed in the coiling assembly, and then fed directly to the encapsulation assembly in which the coil spring is inserted between two sheets of a weldable fabric, the two sheets then being sealed together to form a pocket which encapsulates the spring. The fabric is then indexed forward, the next spring encapsulated, and so on. In this way, lengthy strings of pocketed coil springs may be built up.
It is sometimes desirable to heat-treat the coil springs before they are encapsulated within pockets of fabric. This is because coil springs that have been heat-treated tend to have a reduced loss of height over prolonged periods of use, ie increased resistance to metal fatigue, and tend to be firmer, and in particular more resilient, than coil springs that have not been heat-treated.
However, conventional methods of manufacturing pocketed coil springs that include a heat treatment step suffer from numerous disadvantages. In particular, each coil spring is typically clamped at each end by an electrode, and then heated to approximately 250° C. by resistive heating. This heat treatment step is often the slowest step in the manufacturing process, and hence manufacture is slowed down significantly by the inclusion of a heat treatment step. Furthermore, it is generally necessary to clamp shaped electrodes to each end of the spring, and the electrodes generally need to be adjusted when coil springs are to be manufactured with a new spring geometry. This significantly increases the time taken to change the geometry of the coil springs.